More than 20 years have passed since air-conditioning manufacturers first began marketing year-round electric heating and cooling systems for home and commercial use, under the now-familiar name of heat pumps. The first heat pump units were essentially conventional air-conditioning units which had been modified for reverse operation to pump heat into the building in winter and out of the building in summer. After years of development, heat pump units are now available which are reliable and effective over a relatively wide range of outdoor ambient temperatures, down to about 25.degree. to 35.degree. F outdoor ambient temperature. However, even with higher fossil fuel prices today, presently available heat pump units are usually not the most economically efficient means for residential and commercial heating. This is particularly so for the lower outdoor ambient temperatures in the 10.degree. to 35.degree. range where presently available heat pump efficiency is at its lowest and heating demand is at its highest. Supplemental conventional heat sources are therefore required to meet normal heating demands at the lower outdoor ambient temperatures.
Thus, a need exists for a heat pump apparatus which will deliver more heat to a building or other user for a given input power, than has heretofore been found to be practical. Such an apparatus is needed which will increase heat pump efficiency in the heating mode without affecting it in the cooling mode. Moreover, for simplicity and economy, a heat pump is needed which does not require supplemental heating until very low ambient temperatures are reached; say, below 25.degree. F.